Method for growing transparent conductive GaInO3 films by pulsed laser deposition

ABSTRACT

Applicants have discovered that films of conductively doped GaInO 3  grown on substrates by pulsed laser deposition have conductivity comparable to conventional wide band-gap transparent conductors while exhibiting superior light transmission, particularly in the green and blue wavelength regions of the visible spectrum. Substrate temperatures ranged from room temperature to 350° C. in an ambient containing oxygen at partial pressure in the range 0.1 mTorr to 100 mTorr. The preferred laser source was an excimer laser operating in the deep ultraviolet.

This application is a divisional of copending application Ser. No.08/143,813, filed on Oct. 27, 1993.

FIELD OF THE INVENTION

The present invention relates to methods for growing conductive filmsand, in particular, to a method for growing a transparent conductivefilm comprising GaInO₃.

BACKGROUND OF THE INVENTION

A new transparent conductive material comprising GaInO₃ is disclosed inthe concurrently filed application of Robert J. Cava entitled"Transparent Conductors Comprising Gallium-Indium-Oxide" which isincorporated herein by reference. Specifically, GaInO₃ can be doped withaliovalent elements having valence greater than 3 to achieveresistivities of less than 10 milliohm-cm. This material in bulk formexhibits conductivity comparable to conventional wide band-gaptransparent conductors while exhibiting superior light transmission,particularly in the blue wavelength region of the visible spectrum, andenhanced index matching with typical glass substrates. Preferred formsof the material are GaIn_(1-x) M_(x) O₃ or Ga_(1-y) InM_(y) O₃ where0.005≦x, y≦0.12. Preferred dopants (M) are Sn, Si, Ge, Ti orcombinations thereof.

While the bulk material exhibits highly promising qualities, the primaryuse of transparent conductive materials is as coatings on transparentsubstrates such as glass, fused silica, plastic and semiconductors. Thepresent invention is directed to methods for coating this material onsuch substrates in such a fashion as to retain its conductivity andsuperior light transmission qualities. It also pertains to the resultingproducts.

SUMMARY OF THE INVENTION

Applicants have discovered that films of conductively doped GaInO₃ grownon substrates by pulsed laser deposition have conductivity comparable toconventional wide band-gap transparent conductors while exhibitingsuperior light transmission, particularly in the green and bluewavelength regions of the visible spectrum. Substrate temperaturesranged from room temperature to 350° C. in an ambient containing oxygenat partial pressure in the range 0.1 mTorr to 100 mTorr. The preferredlaser source was an excimer laser operating in the deep ultraviolet.

BRIEF DESCRIPTION OF THE DRAWING(S)

The advantages, nature and various additional features of the inventionwill appear more fully upon consideration of the illustrativeembodiments now to be described in detail in connection with theaccompanying drawings. In the drawings:

FIG. 1 is a block diagram showing the steps in growing a transparentfilm of conductively doped GaInO₃ on a substrate;

FIG. 2 is a schematic view of apparatus useful in practicing the processof FIG. 1;

FIG. 3 is a cross section of a product produced by the process of FIG.1; and

FIG. 4 is a graphical display of absorption versus wavelength for atypical film made in accordance with the process of FIG. 1.

It is to be understood that these drawing are for illustrating theconcepts of the invention and, except for the graph, are not to scale.

DETAILED DESCRIPTION

Referring to the drawings, FIG. 1 is a block diagram showing the stepsin growing a transparent film of conductively doped GaInO₃ on asubstrate. As preliminary steps, a target of conductively doped GaInO₃is provided, as well as a substrate onto which the film is to be grown.The substrate can be glass, fused silica, plastic or semiconductor suchas monocrystalline silicon. The target is conveniently a 1" diameter,1/8" thick pellet of Ga_(1-x) Ge_(x) In_(1-y) Sn_(y) O₃ prepared bysolid state reaction as described in the aforementioned application ofR. J. Cava.

An exemplary suitable target can be prepared as follows. Powderedgallium oxide, Ga₂ O₃, indium oxide, In₂ O₃, and tin oxide, SnO₂, areweighed out to yield the appropriate mole ratios for GaIn_(1-x) Sn_(x)O₃. For instance, for a 5-gram total weight sample of GaIn.sub..94Sn.sub..06 O₃, 2.0090 grams of Ga₂ O₃, 2.7972 grams of In₂ O₃ and 0.1938grams of SnO₂ are used. The powders are mixed together and groundtogether in a mechanical mortar and pestle for a minimum of five minutesto insure good mixing and contact of powder particles. The mixed powdersare transferred into high density aluminum oxide crucibles with coverand heated in air for an initial period of 12-15 hours at temperaturesbetween 1100° and 1200° C. in air. The resulting powders are then groundmechanically again, re-introduced into the covered aluminum oxidecrucibles, and heated for a period of 12-15 hours at 1300° C. in air.After a third grinding, pellets are pressed in a standard steel dye(typically to a load of 3000 pounds for a half-inch diameter pellet).The pellets are then heated again at 1300° C. in air for a period of upto 60 hours. Pellets are typically placed on powder of their owncomposition inside the covered aluminum oxide crucibles for the pelletfiring step. The pellets are cooled after this heating step at thenatural cooling rate of the furnace, which in our case results inreaching 300° C. in approximately three hours, at which point thepellets are removed from the furnace. The resulting pellets aretypically single phase materials (to the detectability limits ofstandard powder x-ray diffraction) of the GaInO₃ structure type. Air isselected as an ambient for the initial processing steps because of itsconvenience. Covers are placed over the crucibles as a precaution toprevent evaporation of the oxides, but we have not observed evaporationunder the conditions described. The first air firing need notnecessarily be in the 1100°-1200° C. range, but we select an initial lowtemperature to reduce the possibility of metastable melting. The finalheating temperature of 1300° C. in air gave more satisfactoryconductivities, in one set of experiments, than did a 1400° C. airtreatment The times and temperatures of these treatments are expected tobe dependent on the reactivities of the starting materials and theefficiencies of the grinding operations. Shorter times can be expectedto give equally satisfactory results in the preparation of ceramictargets for sputtering or laser ablation.

Alternatively, for applications which do not require the highestconductivity, pellets of undoper GaInO₃ after firing can be heated in areducing ambient (e.g. nitrogen-hydrogen (15 mole percent hydrogen) at600°-650° C.). Such targets produce conductive films. It is believedthat they are effectively doped by oxygen vacancies.

The first step shown in block A of FIG. 1 is to dispose the target ofconductively doped GaInO₃ and the substrate in an evacuable chamber. Asshown in FIG. 2, the GaInO₃ target 10 is mounted in chamber 11 on amovable base 12 (preferably rotatable) so that subsequent laser exposuredoes not drill a hole in the target. The substrate 13, which ispreferably transparent material, is mounted on a holder 14 adjacent aheating element 15 for permitting control of the substrate temperature.The target and the substrate are preferably mounted so that the surfaceto be coated faces the target. Exemplary spacing between the target andthe substrate surface is about 8 cm.

The next step illustrated in block B of FIG. 1 is to immerse the targetand substrate in a low pressure ambient (total pressure≦100 m Torr)containing oxygen at partial pressure in the range 0.1 mTorr to 100mTorr and to heat the substrate to a desired temperature in the rangebetween room temperature and 350° C. Temperatures and/or pressuresbeyond these ranges tend to deteriorate the transparency or conductivityof the coating. As a preliminary step the chamber can be evacuated to abase pressure of 1-10×10⁻⁷ Torr, and oxygen is then bled into thechamber. The substrate can then heated to a preferred growthtemperature.

The third step shown in block C of FIG. 1 is to direct upon the targetlaser radiation for evaporating a portion of the target. As shown inFIG. 2 radiation from laser 20 is focussed by lens 21 through chamberwindow 22 onto rotating target 10, thereby ablating material 23 from thetarget onto the substrate 13. Preferably the laser is a KrF excimerlaser operating in the deep ultraviolet region (e.g., 248 nm). In theillustrated arrangement the energy density of the laser on the target isadvantageously adjusted between 1 and 4 J/cm² and the pulse repetitionrate is set at 10 Hz. With this apparatus, material is deposited a rateof 1000-1500 angstroms per hour to a thickness of several thousandangstroms.

After growth, the chamber can again be evacuated, and the coatedsubstrate can be permitted to cool to room temperature. The films emergefrom the chamber transparent to the eye and with good electricalconductivity.

FIG. 3 is a schematic cross section of the finished product showing athin film 30 of polycrystalline, transparent conductive GaInO₃ on atransparent substrate 13. The film preferably has a thickness in therange 0.1 μm to 10 μm.

FIG. 4 is a graphical display of the absorption spectrum for a typicalproduct made in accordance with the process of FIG. 1 (curve 1) ascompared to the absorption spectrum of conventional indium tin oxide(curve 2). The transmission is superior to indium tin oxide in allportions of the visible spectrum, especially in the green and bluewavelength regions.

Such films on transparent glass are useful as windows, selectivelypermitting passage of visible light while reflecting infrared andultraviolet. On glass, silica, or plastic they can be used in a widevariety of optical display devices including active matrix liquidcrystal display devices. And they can be used on semiconductors astransparent contacts for photo-optical devices such as solar cells,lasers, photodetectors and light emitting diodes.

It is to be understood that the above-described embodiments areillustrative of only a few of the many possible specific embodimentswhich can represent applications of the invention. Numerous and variedother arrangements can be made by those skilled in the art withoutdeparting from the spirit and scope of the invention.

We claim:
 1. A method for coating a substrate with a transparent,conductive layer comprising GaInO₃ comprising the steps of:disposing atarget comprising GaInO₃ and a substrate to be coated in an ambienthaving oxygen at partial pressure in the range 0.1 mTorr to 100 mTorr,said substrate disposed adjacent said target for receiving materialevaporated from said target; heating the substrate to a temperature inthe range between room temperature and 350° C.; and shining onto saidtarget pulsed laser light for evaporating a portion of said target ontosaid substrate, thereby coating on said substrate a transparent,conductive layer comprising GaInO₃.
 2. The method of claim 1 wherein theGaInO₃ is doped with an aliovalent material having a valence greaterthan 3 to a resistivity of less than 10 milliohm-cm.
 3. The method ofclaim 2 wherein said aliovalent material is tin, germanium, silicon,titanium or a combination thereof.
 4. The method of claim 1 wherein theGaInO₃ is undoped and deficient in oxygen.